If you put something in a cold pan, it steams/boils itself on the outside and cooks unevenly while the pan or oven is heating up. This almost always results in

If you put something in a cold pan, it steams/boils itself on the outside and cooks unevenly while the pan or oven is heating up. This almost always results in bland flabby overcooked sad food. The lower the thermal conductivity of the food, the more this matters. The shorter the anticipated cooking time, the more this matters. With certain vegetables it's OK to do this, like peppers and onions, because the lower heat "warmup" phase actually helps them soften up and break down, and they are also pretty thermally conductive (lots of water, not much hard fiber). But if you do it with mushrooms you'll just get spongy wet mushrooms, and if you do it with meat you'll get sad boiled stuff instead of something juicy inside and crispy outside. In addition, some cooking techniques specifically rely on the rapid denaturing of proteins on the surface of the food, forming a kind of crust, to prevent it from fusing to the cooking surface. This is how pan-frying fish, meat, and eggs works.